How to use webClient for Basic Authentication

I need to access a webservice from a client to authenticate and get an access token, when I use credentials via java or google arc everything works perfectly and I get the access token but in C # I'm not getting it, it returns error 401 unauthorized always.

below the code I'm using.

using (WebClient client = new WebClient())
{
   client.UseDefaultCredentials = true;
   client.Headers.Add("Authorization", "Basic " +  
        Convert.ToBase64String(Encoding.UTF8.GetBytes("myUser:MyPassword")));
   string response = client.UploadString("https://api.myclient.us/authenticate", 
   "");
   return response;
}

I tried to change this code with several examples on the net but I still have the same error, do I have to change something in web.config, or something in the project?